多表删除，删除一个表的同时删除中间表
1 DELETE u.*,ur.* from t_user u LEFT JOIN t_user_role ur on u.id=ur.userid 2 LEFT JOIN t_role r on r.id=ur.roleid 3 where u.username="张三"
1 DELETE u.*,ur.* from t_user u LEFT JOIN t_user_role ur on u.id=ur.userid 2 LEFT JOIN t_role r on r.id=ur.roleid 3 where u.username="张三"
